Among the known equipment or monitoring the application of the neutral electrodes automatically during high frequency electro-surgery and providing an alarm or a shut-off in certain cases is the equipment disclosed in published German Patent Application PA 15 7021-24 bl of Apr. 2, 1951. In that system upper and lower tolerance limits are set for the resistance interposed between the two neutral counter electrodes, as measured by the test or monitoring current, and when one of these limits is passed, a signal is displayed and/or the high frequency generator is automatically shut-off. The applicable industrial standard, DIN IEC 601, Part 2--2 does not specifically require dual electrodes or monitoring circuits, but requires neutral electrodes of relatively large surface for application to the patient's body in order to provide a return path for the high frequency current at so low a level of current density that physical effects such as undesired burning are avoided. The neutral electrode is required to be applied with its entire surface reliably bearing against the body of the patient. This requirement is backed-up by official rules for high frequency surgical devices in DIN 57 753 Part I, by which attention is required to assure secure provision of contact of the neutral electrode during the entire duration of the application of high frequency current.
The practical use of the monitoring device of the above-mentioned German patent application involves several problems. As a result of more or less individual differences in the nature of the skin of the patient, the electrical resistances to be monitored vary a great deal from patient to patient, so that only insufficiently safe conclusions can be made, from the measured resistance, regarding the effectiveness of the contact surface between patient and neutral electrode. In the case of patients with dry skin the electrical contact resistance is usually remarkably greater than is the case with patients with moist skin. Furthermore, there are today many types of neutral electrodes, for example conductive or capacitive and of different surface materials such as electrically conductive plastic, gelated or dry electrodes, etc. Here again different resistance values appear, so that the equipment according to a PA 15 70 21-24 bl must be separately calibrated for each type.
The problem of the differences in the nature of the skin among various patients is intended to be dealt with by German patent DE 32 39 640 C2 disclosing a monitoring equipment which contains an adaptor unit which controls the setting of the upper and lower limits of the resistance to be measured, so that the upper limit of the electrical signal is set to depend upon the particular value of the normal electrical signal in the particular case. The system is then adapted to monitor changes of the impedance between the two electrode elements during the course of a particular treatment.
These monitoring devices have the disadvantage that the provision of sufficient safety with regard to the application of the neutral electrode to the patient depends upon how well the neutral electrode was applied to the patient before the measurement of the initial value of the impedance between the two mutually insulated electrode elements. If the neutral electrode is applied deficiently to the patient so that one or both electrode elements is or are only partly in electrically conducting contact with the skin of the patient, the upper and lower limits of the impedance would be determined from this defective initial impedance value. In addition, this equipment does not take into account the above-mentioned multiplicity of types of neutral electrodes available on the present market, so that here again the particular type of neutral electrode comes into the question as well as the characteristics of the skin of the patient.